Failure During Fabrication of an Armature Because of Lamellar Tearing

During the final shop welding of a large armature for a direct-current motor (4475 kW, or 6000 hp), a loud bang was heard, and the welding operation stopped. When the weld was cold, nondestructive evaluation revealed a large crack adjacent to the root weld. Investigation showed the main crack had propagated parallel to the fusion boundary along the subcritical HAZ and was associated with long stringers of type II manganese sulfide (MnS) inclusions. This supported the conclusion that the weld failed by lamellar tearing as a result of the high rotational strain induced at the root of the weld caused by the weld design, weld sequence, and thermal effects. Recommendations included removing the old weldment to a depth beyond the crack and replacing this with a softer weld metal layer before making the main weld onto the softer layer.